Bump syn from 2.0.29 to 2.0.31 (#770)

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.29 to 2.0.31.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@2.0.29...2.0.31)

---
updated-dependencies:
- dependency-name: syn
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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